Lack of response to family selection for directional asymmetry in Drosophila melanogaster: left and right are not distinguished in development.

Family selection for directional asymmetry in the expression of the Drosophila melanogaster mutant scute had no result. Fluctuating asymmetry did not show a selection differential correlated with directional asymmetry. The unfolding of bilateral symmetry in embryogenesis can be used to explain the lack of genetic variation for directional asymmetry. Directional asymmetry provides a well-understood example of a developmental constraint in evolution. It is proposed that as no evidence is available for an independent left-right gradient in the embryo, quantitative traits can only be expressed variably along an existing gradient of positional information or a morphogen.